Digital terrestrial broadcasting has started recently thanks to progress in TV technology. A major modulation scheme for digital broadcasting, especially for better resilience to building-induced ghosting interference (also often termed “fading” or “multipaths”), is orthogonal frequency division multiplexing (“OFDM”), which utilizes a multi-carrier system.
OFDM modulation/demodulation is a digital modulation/demodulation scheme utilizing numerous (approximately 256 to 1024) sub-carriers in a single channel bandwidth for efficient transmission of video/audio signals. The OFDM modulation/demodulation performs Inverse Fast Fourier Transform (“IFFT”) on all carriers to generate OFDM-modulated base band (“BB”) signals. A period of an IFFT window is an effective symbol period ts which corresponds to an Fs clock N period. The sum of all digitally modulated carriers with the effective symbol period ts as a base unit is termed an OFDM transmission symbol.
An actual transmission symbol is generally constituted by an effective symbol to which a period tg termed a guard interval (GI) is added, as illustrated in FIG. 11. A waveform for this GI period tg is a repetition of a signal waveform for an end segment 200 of the effective symbol period ts. Therefore, a symbol period length of the transmission symbol is the sum of the effective symbol period ts and the GI period tg. FIG. 11, relating to conventional art, illustrates the symbol period length of the transmission symbol.
ARIB STD-B31, a standard for digital terrestrial broadcasting entitled “Transmission System for Digital Terrestrial Television Broadcasting,” defines the effective symbol period length by means of a basic parameter termed a mode (MODE) as listed in FIG. 12. The standard also defines the GI period (μs) by means of a parameter termed a GI period length (GI ratio) which is a ratio to the effective symbol period lengths as listed in FIG. 13. FIG. 12, relating to conventional art, shows a relationship between (i) a type of mode and (ii) an effective symbol period length corresponding to the mode. FIG. 13, relating to conventional art, shows a relationship between (i) a GI ratio and (ii) a GI period length for different modes.
A collection of a plurality of transmission symbols is termed a transmission frame. More specifically, the transmission frame is constituted by approximately 100 transmission symbols, symbols for frame synchronization, and symbols for service identification. For example, according to the basic parameter of the ARIB STD-B31 entitled “Transmission System for Digital Terrestrial Television Broadcasting”, one frame is constituted by 204 symbols.
According to the basic parameter, the carriers illustrated in FIG. 14 are allocated to each segment in one transmission symbol which has been QPSK (Quadrature Phase Shift Keying)-modulated, 16QAM (Quadrature Amplitude Modulation)-modulated, or 64QAM-modulated. That is, the following types of carrier are prepared: a data signal; an SP (Scattered Pilot) signal; a TMCC (Transmission and Multiplexing Configuration Control) signal; and an AC1 (Auxiliary Channel) signal. Further, (i) a signal intensity and (ii) the numbers of carriers respectively set for the mode (MODE) 1 to the mode (MODE) 3 are assigned to each of the carriers, according to its type. As illustrated in FIG. 14, the total number of carriers set for the mode (MODE) 1 is 108; that for the mode (MODE) 2 is 216; and that for the mode (MODE) 3 is 432. FIG. 14, relating to conventional art, shows (i) a relationship between a type of carrier and a signal intensity corresponding to the type and (ii) a relationship between the type of carrier and the numbers of carriers for different modes.
The SP signal is a pilot signal which is periodically inserted once every 12 carriers in a carrier direction and once every 4 symbols in a symbol direction. The TMCC signal is a signal including a frame synchronization signal, a transmission parameter, and the like. The AC1 signal is a signal including additional information. Unlike the SP signal, the TMCC signal and the AC1 signal are non-periodically inserted to carriers.
Incidentally, as well as conventional analog broadcasting, digital terrestrial broadcasting is required to set in advance a reception frequency, for example, by performing a channel search in an initial state. In digital terrestrial broadcasting, a channel search is performed as follows: After a broadcast signal is received, frame synchronization is established. Then, transmission control information such as TMCC information is extracted. Upon the extraction, a channel search is started. Therefore, there has been such a problem that it takes time to perform a channel search.
In order to deal with this, Patent Literature 1 (Japanese Patent Application Publication, Tokukai, No. 2005-328136 (Publication Date: Nov. 24, 2005)) discloses a channel detecting device including (i) periodic signal detecting means for detecting a periodic signal from a broadcast wave, (ii) periodic distribution determining means for determining a distribution state of a group of the periodic signals, and (iii) determining means for determining, in a case where the group of the periodic signals is within a predetermined width, that there is a digital signal in a certain channel. The channel detecting device disclosed in Patent Literature 1 allows for omission of a step of “confirming a completion of frame synchronization”, which step takes most time in a broadcast channel detecting process. Further, the channel detecting device disclosed in Patent Literature 1 predicts the presence of a channel by referring to the distribution state of the group of the periodic signals which are detected. Thereby, the channel detecting device disclosed in Patent Literature 1 allows for a high-speed channel search.
Patent Literature 2 (Japanese Patent Application Publication, Tokukai, No. 2005-348018 (Publication Date: Dec. 15, 2005)) discloses a digital broadcast receiving device which performs a channel search (i) during a determination process for determining, out of three modes having different OFDM transmission symbol lengths, which mode a received broadcast signal belongs to or (ii) during a determining process for determining a ratio of a guard interval to an effective symbol period.
The digital broadcast receiving device disclosed in Patent Literature 2 performs a channel detection with respect to a reception signal in a time region which is not subjected to fast Fourier transform (“FFT”) yet (i.e., a region which is not converted into a signal in a frequency region yet). This reduces time taken for a channel search.
However, the conventional arrangements arise such a problem that it is impossible to efficiently and quickly perform a channel search with respect to digital terrestrial broadcasting.
Specifically, with the channel detecting device disclosed in Patent Literature 1, in a case where it is determined whether or not there is a digital signal in a certain channel, it is required to detect a periodic signal a plurality of times so as to determine a distribution state of the periodic signals thus detected. This arises such a problem that it takes time for determining whether or not there is a digital signal in a certain channel.
Further, the digital broadcast receiving device disclosed in Patent Literature 2 performs a channel detection process on received channels including channels which clearly do not contain a broadcast wave. This arises such a problem that it is impossible to perform a channel search efficiently.